Water treatment plants, including those that treat surface water sources must generally meet various government requirements with respect to both the effluent distributed from the plant as well as intermediate by-products that may be created during the treatment process. In the United States, water treatment plants are required by the Environmental Protection Agency (EPA) to reduce the total organic carbon (TOC) concentration using a process of coagulation prior to disinfection of the finished water with halogenated/chlorinated disinfectants.
While various tools or instruments for monitoring TOC have been developed, online monitoring of TOC alone does not provide necessary information on the aromatic properties of the sample that are needed to determine the effective coagulation and disinfection dosages needed to prevent formation of toxic disinfection by products (DBPs). The aromaticity is the primary characteristic of the TOC that determines the chemical reactivity with halogenated disinfectants that results in toxic, carcinogenic DBPs.
Current convention uses separate measurements of the absorbance at 254 nm (A254 nm) and TOC concentration with separate instruments/detectors for the purposes of evaluating the effectiveness of the coagulation using what is known as the specific UV absorbance calculation, SUVA=A254 (m−1)/TOC (mg/l). The TOC and SUVA techniques do not provide a reproducible evaluation for different water sources because the aromatic properties of the organic composition of the source water often varies for a particular source over time, as well as among different sources. Further, the SUVA parameter is often imprecise due to the lack of kinetic simultaneity of the parameters of the TOC meter and absorbance detectors, as well as inherent propagated noise/interferences of the separate detection methods as conventionally implemented. Both TOC and A254 are prone to interferences of several types. Use of independent fluorescence data provides a means of ameliorating the influence of primary interferences.
While previous regulations for water treatment plants in the United States required averaged readings of disinfection byproduct formation levels of an entire distribution system, more recent EPA regulations require monitoring of local averages of disinfection byproduct formation levels in different regions of the distribution system. Monitoring only system wide averages may not be sufficient to detect local regions with higher propensity to form DBPs, which may be in violation of the more recent regulations outlined in the EPA Disinfection Byproduct Rule 2 (DBPR2). This of course heightens the need for more precise and accurate TOC and aromaticity evaluations for local regions of the treatment processes.
The imprecision of A254, TOC and SUVA as calculated using EPA specified methods (such as EPA Method 415.3) may also be attributed to the fact that both detector readings are aggregate, single point readings and therefore lack qualitative information on the effects of the coagulation treatment with respect to reactive organic species. As previously described, a number of interferences or confounding factors must be considered with respect to their effect on the readings, including inorganic carbon, metals like iron, and unknown contaminants that may or may not fluoresce. In addition, online TOC meters are highly prone to falling out of calibration, as are online DBP meters (gas chromatographs). Most water treatment plants cannot afford to install and maintain these pieces of equipment. However, as a result of the recent regulatory requirements, many water treatment plants in the United States are considering major infrastructure changes (tens of millions of dollars) including addition of ozone-destruction and ion exchange processes, such as the MIEX resin treatment process, for example.
Commercially available fluorescence instruments have been developed to facilitate parallel fluorescence and absorbance readings. Similarly, laboratory quality instruments are now commercially available to provide nearly simultaneous collection of absorbance and emission data for fluorescence reabsorbance correction. To provide the accuracy and flexibility for a wide variety of applications, these instruments may incorporate separate detectors for absorbance and fluorescence measurements with associated complex optical paths that incorporate a number of mirrors, beam splitters, lenses, and similar optical components, which results in a sophisticated instrument having an associated cost. In addition, the more complex optical arrangement may not be suitable for applications where frequent disassembly and cleaning of elements is required.